In recent years, a metal oxide exhibiting semiconductor characteristics and called oxide semiconductor has attracted attention as a novel semiconductor material having higher mobility and higher reliability than amorphous silicon, and having uniform element characteristics. Metal oxides are used for various applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors whose channel formation region is formed using such a metal oxide having semiconductor characteristics are known (Patent Documents 1 and 2).